In conventional hydraulic pump/accumulator circuits, the load on the pump is controlled by hydraulic or electrohydraulic valves responsive to fluid pressure at the accumulator. During normal operation, the pump feeds hydraulic fluid to the accumulator and to the system load coupled to the accumulator. When fluid pressure at the accumulator and load reaches the desired maximum pressure level, valves deliver fluid from the pump outlet to the sump bypassing the accumulator and load. Although pump load is reduced, the pump continues to operate, generating noise and consuming energy. In systems where the pump is coupled to an electric motor, electrical energy applied to the motor continues to generate heat at the motor, which must be dissipated. The electric power that turns the shaft is termed real/power. Apparent power is line voltage multiplied by current, and includes both the real power and the out-of-phase current component for establishing magnetic lines of flux. This magnetizing component is needed even when the electric motor is unloaded, and is approximately the same magnitude whether the motor is idling or operating at full load.
It is a general object of the present invention to provide an electrohydraulic pump load control system that reduces overall noise level and power consumption by removing application of electrical power from the motor/pump when accumulator/load fluid pressure reaches the desired level. Another object of the invention is to provide an electrohydraulic pump load control system of the described character in which power surges and transients are eliminated as power is selectively applied to the pump motor.
An electrohydraulic pump load control system in accordance with the present invention includes a hydraulic pump coupled to an electric motor for delivering fluid under pressure to an accumulator that stabilizes pump output pressure while accommodating changes in fluid flow. A pressure sensor is coupled to the accumulator to provide an electrical signal as a function of fluid pressure at the accumulator. An electronic controller applies electrical power to the pump motor, and is responsive to the electrical signal from the pressure sensor for terminating application of electrical power to the pump motor when pressure at the accumulator reaches the desired threshold. If disturbance in the electric power supply can be tolerated, the controller may comprise a relay contact electric motor starter. Preferably, the electronic motor controller controls both application and termination of electrical power to the motor to energize and de-energize the motor at predetermined rates responsive to pressure differential thresholds at the pressure sensor.
Most preferably, the motor and pump comprise an integrated electric motor/hydraulic pump unit in which the motor is cooled by hydraulic fluid that flows through the pump. In an air-cooled motor design, the frequency of starting and stopping the motor is dependent upon temperature build-up in its rotor and stator. In a fluid-cooled motor/pump, as is preferred, the frequency of starting and stopping may be considerably increased because of superior heat dissipation. The reduced total input power requirements and increased frequency of staring an oil cooled electric motor/hydraulic pump unit makes this system attractive in machine tool and other applications.